We will determine the natures of "mutations" (in a broad sense) that cause resistance to purine analogues in cultured, human cells. The effects of physical, chemical and biological factors on mutation rates will also be studied. A related objective is to study single-active-X chromosome regulation in the cells. Resistance to azaguaine usually is associated with a deficiency of hypoxanthine-guanine phosphoribosyl- transferase (HGPRT) activity, which is specified by a gene on the X- chromosome. Resistance to fluoroadenine usually is associated with altered adenine phosphoribosyl-transferase (APRT) activity, specified by a gene on autosome No. 16. Regulatory mutations permit expression of the HGPRT gene that normally is repressed on the inactive X. The causes of PRT deficiencies in analogue-resistant variants and of X- chromosome derepression will be studied by means of enzymology, karyotype analysis, cell complementation and somatic cell hybridization. This work will be useful in developing methods for detecting mutagens and carcinogens. The ability to induce these variations will allow the creation of genetically marked cultures to be used in many kinds of experiments with human cells.